The family of human papillomaviruses (HPVs) consist of more than 100 types (also referred to as subtypes) that are capable of infecting keratinocytes of the skin or mucosal membranes. Over 40 types of HPV are typically transmitted through sexual contact and HPV infections of the anogenital region are very common in both men and women. Some sexually transmitted HPV types may cause genital warts. Persistent infections with “high-risk” HPV types (e.g. types 16, 18, 31, 45)—different from the ones that cause skin warts—may progress to precancerous lesions and invasive cancer, e.g. of the cervix, vulva, vagina, penis, oropharynx and anus. The majority of HPV infections are spontaneously cleared within one to two years after infection. In healthy individuals circulating Th1- and Th2-type CD4+ T-cells specific for the viral early proteins E2, E6 and E7 of HPV-16 as well as E6-specific CD8+ T-cells, migrate into the skin upon antigenic challenge, indicating that successful defense against HPV-16 infection is commonly associated with a systemic effector T-cell response against these viral early antigens. In a minority (˜1%) of infected individuals, HPV infection persists, ultimately resulting in genital neoplastic lesions. Among the high-risk HPVs, HPV16 and HPV18 are the main causes of cervical cancer, together causing about 70% of the cases, and these two types also play a major role in other HPV-induced cancers such as anal and oropharyngeal cancer. Worldwide, HPV is one of the most important infectious agents causing cancer.
Vaccination against HPV is deemed a feasible strategy to reduce the incidence or effects of infection by HPV (van der Burg and Melief, 2011, Curr Opinion Immunol 23: 252-257).
Prophylactic HPV vaccines based on virus like particles (VLPs) formed by the (envelope) protein L1 of the HPV types 16 and 18, are very efficient in the prevention of persistent infection and the associated disease by HPV16 and HPV18. These vaccines are believed to provide sterile immunity via the induction of neutralizing antibodies against the L1 proteins. Addition of L1-based VLPs from additional high-risk HPV types may further increase the breadth of protection conferred by such vaccines.
However, while such vaccines can prevent initial infection (i.e., they result in prophylaxis), there is no evidence of a beneficial effect on established genital lesions caused by HPV16 and HPV18, so they are not considered therapeutic vaccines against HPV (Hildesheim et al., 2007, JAMA 298: 743-53).
Despite the introduction of these prophylactic vaccines, large numbers of people have already obtained or are still at risk of obtaining persistent high-risk HPV infections and, therefore, are at risk of getting cancer. Therapeutic vaccines for the eradication of established HPV infections and associated diseases are an urgent unmet medical need.
Some attempts to address this need have been described. For example, clinical trials have been carried out with a variety of different vaccination strategies, such as a fusion protein consisting of a heat shock protein (Hsp) from Mycobacterium bovis and HPV-16 E7 or a fusion protein of E6, E7 and L2 from HPV-16 and HPV-18, chimeric L1-E7 VLPs, recombinant vaccinia viruses expressing either E6 and E7 of HPV-16 and HPV-18 or bovine papilloma virus E2, DNA vaccines expressing CTL epitopes of E6 and E7 of HPV-16 and HPV-18, a live-attenuated Listeria monocytogenes (Lm) that secretes the HPV-16 E7 antigen, and synthetic long-peptides (SLPs) comprising HPV-16 E6 and E7 peptides. While some of these approaches show some, but limited, clinical efficacy, most have failed, demonstrating that improvement of the current strategies is needed.
Integration of the genes encoding the early HPV proteins E6 and E7 is a necessary step in the process from infection to cancer and continuous expression of E6 and E7 is required for the maintenance of the neoplastic phenotype of cervical cancer cells. E6 and E7 are therefore considered good targets for therapeutic vaccination. As mentioned some studies have shown that therapeutic vaccination of women infected with high-risk HPV can induce regression of existing lesions. Kenter et al showed a durable and complete regression in 47% of patients having Vulvar Intraepithelial Neoplasia (VIN) using SLPs derived from the HPV16 E6 and E7 proteins and an adjuvant as a therapeutic vaccine (Kenter et al., 2009, N Engl J Med 361: 1838-47). Similarly, a study in which a protein-based vaccine (TA-CIN, consisting of a fusion protein of HPV16 E6, E7 and L2) was combined with local immune modulation in VIN 2/3 patients, showed complete regression in 63% of patients (Daayana et al., 2010, Br J Cancer 102: 1129-36). Possible drawbacks of the synthetic long peptides as a vaccine include manufacturability at large scale and costs associated therewith, the need for potentially reactogenic adjuvant and the associated adverse effects associated with immunization (especially pain and swelling). Due to the high level of discomfort it is not likely that SLPs will be used in early stage disease when the spontaneous clearance rate is still high. Similarly, due to the need for local imiquimod treatment in the case of TA-CIN treatment, tolerability is a significant issue as the majority of women experience local and systemic side effects lasting for the duration of imiquimod treatment, which may affect daily activities.
A possible alternative is to use nucleic acid based vaccination such as DNA vaccines or viral vectored vaccines encoding the HPV E6 and/or E7 protein for vaccination.
However, the HPV E6 and E7 proteins have oncogenic potential and thus vaccination with vaccines that comprise nucleic acids encoding these proteins poses a risk of inducing cellular transformation due to the possibility of prolonged expression of the antigens.
Therefore, in case of genetic vaccination, non-oncogenic/detoxified versions of E6 and/or E7 can be used in order to exclude any risk of cellular transformation due to the vaccination. Loss of oncogenic potential of wild-type E6 and E7 is commonly achieved by deletion and/or substitution of residues known to be important for the function of these proteins (e.g., Smahel et al., 2001, Virology 281:231-38; Yan et al., 2009, Vaccine 27: 431-40; Wieking et al., 2012, Cancer Gene Ther 19: 667-74). However, a disadvantage of these approaches is that they carry the risk of removing important T-cell epitopes from and/or introducing new undesired T-cell epitopes into the proteins, and may thus not lead to the desired immune response.
In an alternative strategy to remove the oncogenic potential of HPV16 E6 and E7, shuffled versions (i.e. polypeptides wherein fragments of the wild-type protein are re-ordered) of the E6 and E7 proteins have been constructed (e.g. Öhlschläger et al., 2006, Vaccine 24: 2880-93; Oosterhuis et al., 2011, Int J Cancer 129: 397-406; Oosterhuis et al., 2012, Hum Gen Ther 23: 1301-12). However, these approaches would still require manufacturing, formulation and administration of multiple molecules to ensure inclusion of all possible epitopes of both the E6 and E7 proteins, resulting in sub-optimal logistics and relatively high costs, and moreover the strategies described introduce potentially strong non-natural epitopes that are not present in E6 and E7 and since immune responses could be diverted from relevant E6/E7 epitopes towards such non-natural epitopes, the described constructs may not have the optimal immunological characteristics. A therapeutic DNA vaccine expressing an intracellularly targeted fusion protein with built-in genetic adjuvant and shuffled fragments of E6 and E7 of both HPV16 and HPV18 has also been described, and electroporation-enhanced immunization therewith elicited a significant E6/E7-specific T-cell response in CIN3 patients (Kim et al., 2014).
There remains a need in the art for therapeutic vaccines against HPV, preferably having less of the drawbacks of the approaches described before.